1. Field of the Invention
The present invention relates to a semiconductor light source, and more particularly, to a linear light source for use in an electrographic copying machine and a facsimile machine.
2. Description of the Prior Art
A linear light source including a plurality of semiconductor light emitting devices arranged linearly is frequently used as an original irradiating light source and as a charge removing light source in an image forming apparatus such as an electrographic copying machine and a facsimile machine. A general arrangement of the linear light source is such that the semiconductor light emitting devices are arranged on a long and narrow base plate and used in combination with a lens for condensing light emitted from the semiconductor light emitting devices.
Referring to FIG. 1, there is shown a conventional linear light source. On the surface of a base plate 102, light emitting diodes 103 are arranged in a line with predetermined spaces. Above the light emitting diodes 103, a bar lens 104 is arranged for condensing light emitted from the light emitting diodes 103. The bar lens 104 has both ends fixed to holders 106. On both sides of the line of the light emitting diodes 103, reflecting frames 105 are arranged to reflect scattered light toward the bar lens 104. On the base plate 102, reflecting plates 107 are arranged between the light emitting diodes 103. To assemble the conventional light source, first, the light emitting diodes 103 are arranged on the base plate 102, and then, the bar lens 104, the holders 106 and the reflecting frames 105 which are formed separately are fixed. According to this manner, however, a high assembling technique is required to arrange the parts at respective positions.
To solve this problem, Japanese Laid-open Patent Application No. H2-215171 discloses to integrate the lens and the reflecting frames into a frame member for holding therein the base plate on which the light emitting diodes are arranged. A linear light source structured by this method is shown in FIG. 2. The bar lens 104 and the reflecting frames 105 which are made of different resin materials are integrated into a frame member 101 by two-color continuous extrusion method. In the inner surface of the frame member 101, grooves 108 for inserting the base plate 102 therein are formed along the length. The base plate 102 on which the light emitting diodes 103 are arranged is held in the frame member 101 by being inserted in the grooves 108.
In the linear light source of this structure, since the base plate 102 is held in the frame member 101 while being substantially in close contact with the reflecting frames 105 and since the reflecting frames 105 and the bar lens 104 are integrated, it is unnecessary to adjust the positions of the base plate 102 and the bar lens 104, thereby simplifying the assembling process. In addition, since there are no gaps through which dust such as paper dust intrudes, the interior does not become dirty.
However, since the base plate 102 is surrounded on its upper, lower, right and left sides by the frame member 101, it is impossible to release the heat generated when the light emitting diodes are activated. For this reason, the temperature of the base plate 102 becomes high, so that the base plate 102 warps along the length due to thermal expansion. Since the base plate 102 is substantially in close contact with the grooves 108 of the frame member 101, the warp of the base plate 102 works on the frame member 101 to warp it. The frame member 101 warps easily since thermal expansion is not uniform owing to the fact that the frame member 101 is thin at its base 109 whereas it has the thick bar lens 104 at its top. The heat accumulated inside the frame member 101 is transmitted to the frame member 101 to increase the temperature thereof, so that the frame member 101 warps. If the frame member 101 warps, the distance from the light source to the light irradiated surface will become non-uniform. Then, the irradiation width and the illuminance will become non-uniform. This means that the performance of the linear light source deteriorates.
The base plate 102 is in contact with the frame member 101 at both sides of its upper surface, at its both sides and at the entire of its bottom surface. Thus, the base plate 102 and the frame member 101 are in contact in a large area. In assembly, the base plate 102 is inserted into the groove 108 from one end of the frame member 101. However, since the frictional resistance is large because of the large area of contact, the base plate 102 may not be inserted smoothly. The insertion of the base plate 102 is facilitated by forming the groove 18 to be slightly larger than the base plate 102. However, the base plate 102 still warps because of the increase in temperature. In this case, the distance between the light emitting diodes 103 and the bar lens 104 becomes non-uniform. If the distance becomes non-uniform, the irradiation width of the light condensed by the bar lens 104 will become non-uniform, and the illuminance will also become non-uniform.
To supply power to the light emitting diodes, it is necessary to arrange a pair of conductive patterns on the base plate. In a very long linear light source, a problem arises in arranging the conductive patterns. The conductive patterns are provided on both sides along the line of the light emitting diodes 103. Generally, each of the light emitting diodes is placed on an electrode connected to the first conductive pattern, and connected to the second conductive pattern by a bonder by using a metallic fine wire. However, the size of the base plate is normally limited to A3 size paper sheets or smaller because of the capability of the bonder. A linear light source longer than that is formed by linking a plurality of base plates along the length. In this case, the pair of conductive patterns is required in a number corresponding to the number of base plates, and it is necessary to arrange the necessary number of conductive patterns at both sides of the base plates. This increases the width of the base plates. The longer the linear light source is, i.e. the more number of base plates are linked, the wider the base plates are to increase the size of the light source.
It is desirable for a linear light source to have an illuminance which is high and uniform along the entire length. Since the light emitted from the light emitting diodes proceeds toward the sides as well as toward the lens, the quantity of light used for irradiation is limited. To reflect the light proceeding to the sides toward the lens, reflecting frames are frequently used which are formed to have inner surfaces inclining so that the width of the interior increases from the base plate toward the lens. The quantity of irradiated light is also increased by arranging the reflecting plates 107 as shown in FIG. 1, or by applying a reflecting coat on the upper surface of the base plate on which the light emitting diodes are arranged to thereby reflect the scattered light. Further, it is known from Japanese Laid-open Utility Model Application S63-131157 that the quantity of available light is increased by covering the surfaces of the light emitting diodes with a coating resin. These methods are all effective in increasing the illuminance and the illuminance is largely increased by using them in combination. However, by these methods, it is difficult to maintain the uniformity of the illuminance.
For example, a light source as shown in FIG. 3 was formed in a manner such that a reflecting coat 110 was applied onto the base plate 102, the light emitting diodes 103 were covered with a coating resin 111 and the reflecting plates 107 were arranged between the light emitting diodes 103. Although the illuminance improved as a whole, the illuminance was non-uniform along the length. The illuminance characteristic of the light source is shown by a broken line in FIG. 16. The numerals on the horizontal axis show the locations of the light emitting diodes 103. As shown in the figure, the illuminance decreases greatly in the spaces between the light emitting diodes 103 compared to where the light emitting diodes 103 are present. In addition, the highest illuminances at the places where the light emitting diodes 103 are present are slightly different from one another, and the ranges where the illuminance is high are not the same. The variation in illuminance at the places where the light emitting diodes 103 are present is attributed to the non-uniform configuration of the coating resin 111 covering the light emitting diodes 103.